Comments

English doesn't use diacritics very often, and certainly not over the letter a. It did use diacritics more in the past, but they have been almost entirely expunged from the language. The are a few left I can think of such as naïve, café, résumé, even now most people simply leave them out. Most of them are of French origin. French diacritics would probably be the only ones that English speakers might be familiar with.

What I would suggest you do instead is change the spelling to Yahna if that is how it is pronounced. If it sounds more like a short o than an open ah (as in Father), then spell it Yonna - like Donna.